1. Field of the Invention
The present invention relates in general to the wireless telecommunications field and, in particular, to a processing unit and method for using a logistic function to map a score output from an objective voice quality method (e.g., Perceptual Evaluation of Speech Quality (PESQ) method) so that the mapped score corresponds to a mean opinion score (MOS) that is an estimation of the subjective quality of a speech signal transmitted through a wireless network.
2. Description of Related Art
Manufacturers and operators of wireless networks are constantly trying to develop new ways to estimate the voice quality (e.g., to estimate the mean opinion score (MOS)) of speech signals transmitted through a wireless network. Today the manufacturers and operators use an objective metric defined in the International Telecommunication Union, recommendation ITU-T P.862, to estimate the subjective quality of a speech signal transmitted through a wireless network. The ITU-T P.862 recommendation is entitled “Perceptual Evaluation of Speech Quality (PESQ), an Objective Method for End-to-End Speech Quality Assessment of Narrowband Telephone Networks and Speech Codecs”. The contents of ITU-T P.862 are incorporated by reference herein. Although the score from the PESQ has a high correlation with the subjective MOS it is not on exactly the same scale as the subjective MOS which is measured in a subjective test by listeners performed in accordance with ITU-T recommendations P.800 and P.830. The PESQ score is between −0.5 and 4.5 while the subjective MOS score is between 1.0 and 5.0. As such, a PESQ score of below 2.0 corresponds to “bad” quality while “bad” quality for MOS is usually below 1.5. This difference in scales is problematical in that the score from the PESQ algorithm is not suitable for field measurement tools. Accordingly, there have been several attempts to address this problem by developing mapping functions to map a PESQ score to the MOS domain like the Auryst mapping functions described below and like the mapping functions described in the following articles the contents of which are incorporated by reference herein:                NTIA, ITU-T Study Group 12, delayed contribution D-029, April 1997, “Additional Detail on MNB Algorithm Performance”. This contribution was subsequently published in IEEE Transactions on Speech and Audio Processing, Vol. 7, No. 4, July 1999.        Irina Cotanis “Impacting Factors on the Objective Measurement Algorithms for Speech Quality Assessment on Mobile Networks”, IEEE International Conference on Telecommunications, Bucharest Romania June 2001.        Psytechnics Ltd., ITU-T Study Group 12, Study Period 2001, delayed contribution D.86, “A New PESQ-LQ Scale to Assist Comparison Between P.862 PESQ score and Subjective MOS”.        Timothy A. Hall “Objective Speech Quality Measures for Internet Telephony”, in Voice over IP (VoIP) Technology, Petros Mouchtaris, Editor, Proceedings of SPIE Vol. 4522 (2001).        Christopher Redding et al. “Voice Quality Assessment of Vocoders in Tandem Configuration” NTIA Report 01-386 April 2001.        Stephen D. Voran “Objective Estimation of Perceived Speech Quality Using Measuring Normalizing Blocks” NTIA Report 98-347 April 1998.        Stephen D. Voran “Objective Estimation of Perceived Speech Quality, Part I: Development of the Measuring Normalizing Block Technique”, IEEE Transactions on Speech and Audio Processing, Vol. 7, No. 4, July 1999.        British Telecom, ITU-T Study Group 12, delayed contribution D.79 “Performance Metrics for Objective Quality Assessment Systems in Telephony” dated December 1998.        British Telecom, ITU-T Study Group 12, delayed contribution D.80 (December 1998) “Comparison of Speech Quality Assessment Algorithms: BT PAMS, PSQM, PSQM+ AND MNB” dated December 1998.        A first release of Auryst's mapping function originally developed by LCC International and subsequently purchased by Ericsson, used a mapping from the raw output values to dBQ and thence from dBQ to MOS. And, the second release of Auryst's mapping function used a logistic function that had parameters a, b, c and d optimized as:        
  y  =      a    +                  b        -        1                    1        +                  ⅇ                                    c              ·              x                        +            d                              
Many of these mapping functions do not work well for one reason or another. For example, the mapping functions described in the four articles by Timothy A. Hall, Christopher Redding and Stephen D. Voran where the output is mapped to the 0 to 1 range. Even though some of these mapping functions work well, such as the second release of Auryst's mapping function, there is still a need for improvement especially for wireless applications. This need is satisfied by the mapping (logistic) function of the present invention.